The purpose of this research is to ascertain if defective tRNAs accumulate during aging and cause errors in protein synthesis. The acylation capacity of ribosome-bound and nuclear tRNAs of old animals will be characterized, since old rat liver tRNAs accepted lesser amounts of amino acids when whole tissue was used to purify the tRNA but accepted normal amounts of amino acids if high-speed supernatant was used. Effects of polyamines on acylation capacity will be studied. The tRNAphe will be purified and its primary structure examined. The effect of halogenatd phenylalanines on acylation of tRNAphe will be explored, to see whether old tRNA accepts more or fewer incorrect acylations under various conditions. The level of in vivo acylation of several tRNAs will be assessed. The effectiveness of young and old tRNAs in promoting the normal diversity of polyeptides made in vitro using Endomyocarditis (EMC) virus messenger RNA and a Krebs ascites tRNA dependent protein synthesizing system will be examined. If old and young tRNAs differ in these structural or functional tests, the old tRNAs will be modified in vitro and reassayed to see whether modification deficiencies could account for the changes.